The method and system are generally related to maskless lithography or direct-write digital image technologies. More specifically, it relates to a double-sided maskless exposure system capable of simultaneously exposing both surfaces of a subject plate, such as a substrate for a printed circuit board (PCB) or a sheet for lead frames. The system disclosed herein could be used to create double side exposures for PCB, IC packaging and LCD manufacturing. It could also include applications such as document printing and photographic reproduction etc. The following description focuses on PCB exposure equipment, although the specification can be applied by obvious extension to other fields of use as well.
The current PCB exposure industry mainly uses film masks. The technology suffers many disadvantages, e.g., film deformation, low alignment accuracy, limited line width around 4 millimeter, and difficulties in film storage and management etc. With the PCB industry moving towards high-density interconnection (HDI) board, multi-layered board and other trends, along with rising demands for high precision alignment, the traditional film mask (Mask) exposure lithography process is limited by the technical production bottleneck. To solve the yield and productivity problems, the PCB industry has paid more attention to the emerging maskless lithography technology or the direct imaging equipment (Direct-write Digital Imaging System). The maskless lithography technology is expected to grow into a mainstream technology in lithography. Conventional film mask lithography for PCB is relatively cheap but cannot overcome many problems such as the distortion between PCB layers and the scaling issue. The development of HDI multilayer and high-density boards highlights the many advantages of using the maskless technology. The technology enjoys at a high exposure speed on the traditional dry film photo-resist with distortion correction. A direct-write digital imager can achieve high output (high throughput), high yield, yet with the lowest overall cost. This new kind of PCB maskless exposure equipment is gaining popularity in the industry.
The method and system also relates to double-sided exposure systems examples of which include U.S. Pat. No. 5,337,151, U.S. Pat. No. 5,627,378, U.S. Pat. No. 5,923,403, U.S. Pat. No. 5,929,973, U.S. Pat. No. 5,933,216 and U.S. Pat. No. 6,211,942. The technology requires masks for imaging patterns onto photo resist coated sides of a subject. The subject plate may include, for example, a semiconductor substrate for manufacture of integrated circuits, metal substrate for etched lead frame manufacture, conductive plate for printed circuit board manufacture, or the like. A patterned mask or photo mask may include, for example, a plurality of lines, structures, or images. With conventional photolithography, the patterned masks or films for high resolution applications are typically very expensive and have a short lifetime. In addition, photomasks often require a long mask purchase lead time. The long mask purchase lead time creates a roadblock when a short product development cycle is desired. Further, when a particular mask design requires a design change in the pattern, regardless the size of the change, it requires a long lead time and associated mask modification cost. Frequent mask modifications can cause serious problems in PCB manufacturing.
A double-sided maskless exposure system is also advantageous over single side maskless exposure systems. Most PCBs need exposure on both sides. A single side maskless exposure system doubles the exposure for exposure on both sides of the board and requires additional alignment process to align the patterns on both sides. Poor alignment often reduces system productivity and yield. A double-sided maskless exposure system, however, does not require pattern alignment and is compatible with the conventional double-sided exposure equipment and other processes. It works especially well for flexible exposure subjects such as a lead frame. A lead frame is fed in a roll. The continuous exposure required for a lead frame make it difficult to use a single side maskless exposure system due to required pattern alignment process for both sides. It would require multiple pieces of equipment, which comes with higher cost.
A double side maskless system (U.S. Pat. No. 6,396,561, US2009/0279057) is apparently a better choice than a single side maskless exposure system. The key is how to achieve desired system stability and reliability after adding another side maskless exposure mechanism. There are at least two maskless optical engines in a double-sided exposure system, the distance between them is often too far to get accurate alignment of the two optical engines. Even if the maskless optical systems were aligned in fabrication, it could easily change due to vibration, temperature change, and other environmental condition. An auto-calibration system or self-check function is necessary to ensure the alignment of both sides and exposure quality.
There are currently a few types of double-sided maskless exposure systems in the market. One uses a laser beam scanning on a substrate surface; another is called direct imaging, which uses a 2D Spatial Light Modulation (SLM), such as Digital Mirror Device (DMD) to project a pixel array on a substrate. The system disclosed herein relates to the direct imaging method. When using the direct imaging method, each of the plurality of pixel elements of SLM is simultaneously focused on portions of the subject plate. The subject and pixel elements are then moved (e.g., by vibrating one or both the subject and pixel elements), and the sub-pattern is changed in response to the movement and the SLM pixel pattern. As a result, UV light can be projected into the sub-pattern to create the plurality of pixel elements on the subject, and the pixel elements can be moved and altered, according to the pixel-mask pattern, to create a contiguous image on the subject. In a 2D direct imaging method, there are three generations of maskless optical engines; the first generation is to directly image a 2D SLM on a substrate with enlarging or shrinking pixel size without any transformation. The second generation is called the point array method, which uses micro-lens to focus the light from each pixel and get the focus points on a substrate surface. The third generation is called sub-image array method, which uses a special optical system to divide the image of the whole 2D SLM into a sub-images array with shrinking pixel size on the substrate. The difference between the three generations is the imaging optical system.